1. Field of the Invention
The present invention relates to a driving unit and a timepiece.
2. Description of Related Art
Conventional driving units include elements, such as piezoelectric elements or electrostrictive elements, which can expand and contract under applied voltage.
Examples of the driving units utilizing the piezoelectric effect of piezoelectric elements include a stacked driving unit driving with expansion/contraction of multiple stacked piezoelectric elements under applied voltage; a bimorph driving unit including two bonded plate piezoelectric elements that can be warped in the direction perpendicular to the longitudinal direction of the piezoelectric elements (i.e., the direction perpendicular to the bonded surfaces of the piezoelectric elements) by the applied voltage causing expansion of one of the piezoelectric elements and contraction of the other piezoelectric element; and a unimorph driving unit including a metal plate with one surface bonded to a plate piezoelectric element and can be warped by expansion/contraction of the piezoelectric element under applied voltage.
Such a driving unit including piezoelectric elements has a relatively small size and light weight. The driving unit is especially advantageous when installed in a small accommodation space of a compact electronic device, such as a watch.
Although the stacked driving unit has strong driving force and is highly responsive, the driving unit requires high voltage and high frequency for the driving, resulting in high power consumption. The unimorph or bimorph driving unit, which includes a single-plate piezoelectric element rather than stacked piezoelectric elements, consumes lower power but outputs weak driving force and is inferior in responsiveness. Eagerly anticipated is a technique to increase the reliability of the driving units for practical use.
For example, Japanese Unexamined Patent Application Publication No. 2000-152671 discloses an ultrasonic motor including ultrasonic oscillators (piezoelectric oscillators) that generate ultrasonic vibrations under applied voltage. The ultrasonic motor disclosed in Japanese Unexamined Patent Application Publication No. 2000-152671 includes drivers each including two ultrasonic oscillators (piezoelectric oscillators) orthogonal to each other, a rotor, pushers to push the drivers against the outer periphery of the rotor, and AC sources to supply the two ultrasonic oscillators with AC signals having different phases. Under voltage applied to the ultrasonic oscillators, the drivers generate ultrasonic vibrations and come into contact with the outer periphery of the rotor at the tips, to rotate the rotor.
The driving unit disclosed in Japanese Unexamined Patent Application Publication No. 2000-152671 includes multiple drivers each including two ultrasonic oscillators (piezoelectric oscillators) around the rotor. Unfortunately, this configuration causes complication and an increase in size of the device.
The invention relates to a driving unit having a relatively simple structure, a small size, and a light weight while achieving stable rotation of the rotor, and a timepiece including the driving unit.